The present invention relates to a system for observing objects in three-dimensional space. More particularly, the invention relates to a multi-camera, three dimensional sensing system for observing an object by disambiguating reflections of multiple light patterns from a sensed object to obtain a set of data points defining the three dimensional shape of the object. This set of data points is then compared with a set of predetermined ideal shape information data points, or a reference model, to identify gauge measurements of the object.
Traditionally, gauge measurement of a manufactured object having a complex three dimensional surface such as an airfoil (e.g. turbine blade) is a tedious and time consuming process. Airfoils, including forged blades such as those used on aircraft engines, electrical power generators, and the like, are inspected for deformations which may include, but are not limited to, skew, twist, scaling, and translation. More specifically, airfoils are inspected for deformation parameters such as platform orientation, contour cross-section, bow and twist along a stacking axis, thickness, and chord length at given cross-sections.
One method of obtaining dense and accurate digital data representing these parameters for an individual airfoil is by use of a coordinate measuring machine (commonly known as a “CMM”). CMM's translate and rotate a sensor probe into contact with the surface of an object undergoing testing, to sample the position of various points on the object's surface. Before a sensor probe is brought into contact with the article under test, the object must first be fixed in a known physical position and orientation, such that a set of known reference points can be established. For airfoil measurement, six physical contact points are utilized, this arrangement being referred to by those skilled in the art as a “six point nest”. Defining this set of six data points enables the position and orientation of the airfoil in its physical holder to be subsequently translated to any other coordinate system. CMM's provide dense measurements of the sample points. However, the time to scan an airfoil is relatively slow because the sensor probe must be continually repositioned to obtain data. After the necessary information on the surface points is collected, computer software processing computes deviations of these points from a computer assisted drawing (CAD) reference model and analyzes the deviations in terms of process-based shape deformations. Current CMM processing software, however, is also relatively slow.
An alternate method for obtaining data on these parameters, where speed is of importance, is by hard gauging using micrometers, calipers, shims and other gauges. In hard gauging, an object undergoing measurement is placed in close proximity to a set of molds or forms configured to exactly match the desired parameters of an ideal object. Comparisons are then made between the surfaces of the object being measured and the set of molds or forms using the mechanical gauges to determine if there are any deviations from the ideal object shape. It will be appreciated that hard gauging is costly since a new set of gauges must be machined for each object undergoing test, and they are inflexible to change. Further, while hard gauging is fast, it does not provide dense measurement data, but rather provides only individual measurements at a relatively few defined contact points.
Yet another method of obtaining measurements representing deformation parameters of an object employs full-field non-contact range sensors. These sensors scan the external surfaces of opaque objects, using laser or white light, significantly faster than CMMs. While the sensors are capable of scanning an object quickly and obtaining large quantities of data, the accuracy of this data is significantly lower than that obtained using CMMs. Examples of non-contact sensors include sensors based on laser line grating and stereo triangulation; and sensors based on single laser line scan plus rotation of the object. Additional non-contact sensors are based on phase-shifted Moiré patterns and white light.
Currently, there is a need for a system capable of obtaining accurate high-speed measurements of the surfaces of an object, efficiently and accurately registering the measurements to a set of reference surface measurements of an ideal object, filtering noise from the registered measurements, identifying surface deformations, and displaying the resultant gauging information to an operator. Further, there is a need for the system to be capable of being rapidly reconfigured to measure one or more differently shaped objects without the need to replace gauge components from one object to the next, if the objects are different.